Pair tunneling as a probe of order-parameter fluctuations in superconductors: Zero magnetic field effects

Thermal fluctuations of the order parameter in a superconductor above its transition temperature result in manifestations of the ordered state which appears below the transition. In a Josephson junction consisting of two metals with different transition temperatures there is associated with the incipient superconductivity an enhanced conductivity across the junction at temperatures between the transition temperatures of the two metals. The enhanced conductivity is observed as a current which flows in excess of the junction quasiparticle current. The excess current has been investigated using Sn-Sn_xO_y-Pb and Al-Al_xO_y-Pb junctions masked to eliminate effects of graded film edges. Details of the variation of the excess current with voltage and temperature are in quantitative agreement with calculations in the Al-Al_xO_y-Pb junctions. The excess current-voltage characteristic is a direct measure of the imaginary part of the generalized susceptibility above T_c. The pair relaxation frequency in aluminum based on the data is consistent with the theoretical value.